1. Field of the Invention
This invention relates to a pressure relief system for a lubrication reservoir for sealed bearing rotary cone rock bits.
More particularly, this invention relates to a two-piece canister for a lubricant reservoir for a sealed bearing rock bit, the reservoir being vented through the dome of the rock bit.
2. Description of the Prior Art
There are a number of prior art references relating to pressure relief systems for lubricant reservoirs for sealed bearing rock bits.
U.S. Pat. No. 4,055,225, issued to Hughes Tool Company, teaches a lubricant pressure compensator system for an earth boring drill bit for equalizing the lubricant pressure with the ambient or borehole pressure. The pressure compensator is located in a lubricant reservoir that has a passage leading out of the bit adjacent the discharge nozzle. A flexible diaphragm is located in the reservoir with its lip sealed against the base of the reservoir. The lip protrudes outwardly and a protector member bears downwardly against the lip. A cap bears against the protective member in pressing the lip. A retainer snap ring holds the cap with the lip under compression. The groove for the snap ring is formed in the body of the rock bit. A protective centering disc is attached to the center of the diaphragm. An O-ring groove is formed in the bit body below the snap ring groove to form a seal around the cover cap. The O-ring, after it is installed within its groove, is subject to damage when the cover cap is forced past the O-ring during the installation phase prior to insertion of the snap ring to retain the cover cap within the lubricant reservoir cavity. Another patent issued to Hughes Tool Company, U.S. Pat. No. 4,276,946, utilizes an identical cover cap retention device that is subject to the same O-ring damage as the foregoing patent.
U.S. Pat. No. 4,161,223, assigned to the same assignee as the present application, describes a manual venting and pressure relief system located within a lubrication reservoir cavity of a sealed bearing rock bit. The lubricant system is provided in each leg of a three leg rotary cone rock bit to provide lubricant to the bearing areas between the cutter and the leg. The lubrication system includes a reservoir of lubricant that is retained within a rubber boot molded around a metal stiffening sleeve. A cover cap is attached to the rubber boot. The boot is in the form of a resilient membrane and is exposed through the cover cap to the exterior of the rock bit. The vent and pressure relief system comprises an annular seat formed in the wall of the reservoir. A valve face is formed on the rubber boot and is biased against the annular seat by means of a belleville spring acting on the cover cap. Any excessive pressure developed during operation of the bit within the lubricant reservoir is blown off through the valve seat. Any internal pressures can also be manually vented, without removing the cover cap, by prying the cap away from its seat.
Still another patent, U.S. Pat. No. 4,388,984, assigned to the same assignee as the present application, teaches a pressure relief system similar to the foregoing system with the difference being that the reservoir system is a two-stage vent system for a rock bit which relieves low pressure gases in a first stage and higher pressure gases in a second stage.
The '223 and '984 references both relieve pressures externally through the reservoir cover cap of the rock bit. The instant invention senses differential pressure across the boot through an opening in the dome of the bit as is taught in both of the Hughes' references.
The present invention teaches the use of a two-piece canister device that protects a packing gland, such as an O-ring seal, while assuring positive sealing during assembly of the pressure relief system within a rock bit. The instant invention also utilizes a dome venting system in combination with the two-piece canister device for the relief mechanism.